The capture of well exposed images (elements with reflectance at 18% of the scene reaching the middle of the dynamic range in the captured visible image) in very low lighting situations in theory requires the use either of an additional source of light of the flash type, which changes the ambiance of the scene, or of photographic equipment equipped with a highly sensitive sensor and with a wide aperture objective lens so as to capture the maximum amount of light which is too bulky to fit into a pocket of a garment.
In both instances, this is supposed to allow the use of an exposure time that is short enough that it limits the movement blur in the image. The generally accepted rule for defining the maximum exposure time T that can be used as a function of the 35 mm equivalent focal length value f is T=1/f (Xiao, F. Pincenti J., John G. and Johnson K., “Camera motion and mobile imaging”, Proceeding of Electronic Imaging (2007)).
The 35 mm equivalent focal length value currently considered as a reference for fixed focus equipment is between 28 mm and 35 mm. The maximum useable exposure time is therefore between 1/28s and 1/35s.
The use of an additional source of light is usually unwelcome because it changes the low-lighting atmosphere of the scene and therefore does not allow the scene being experienced by the individual taking the photo to be recreated.
Photographic equipment equipped with a highly sensitive sensor and with a wide aperture objective lens generally means a reflex camera with interchangeable objective lens.
This type of equipment makes it possible to obtain very high quality images with low noise (high signal to noise ratio (SNR)), a high level of sharpness and texture (high SFR (spatial frequency response) edge and SFR texture values) and a very broad dynamic range.
The noise as used in calculating the SNR can be characterized as measured from the “visual noise” described in D. Baxter, F. Cao, H. Eliasson, J. Phillips. Development of I3A CPIQ spatial metrics. SPIE Conference on Electronic Imaging 2012. This evaluation of the noise allows the variations in luminance and in chrominance in the image to be taken into consideration simultaneously.
The SFR edge and the SFR texture are two measurements of sharpness and of texture. These two measurements are described in the same document under the headings “Edge acutance” and “Texture blur” respectively.
Nevertheless, this type of equipment is heavy and cumbersome, which means that it is not always possible to have it about one's person under all circumstances.
Moreover, the use of a very wide aperture combined with the use of a large sized sensor results in a reduced depth of field, something which on the one hand makes focusing more difficult and on the other hand reduces the number of sharp elements in the image.
Photographic equipment of the reflex camera or DSLR camera type is known in the prior art. This type of equipment has the disadvantage of being bulky and heavy. DSLR cameras make it possible to obtain an excellent quality of image (very good SNR, and very good texture levels) because of the large size of their sensor (diagonals greater than or equal to 22 mm) and the possibility of fitting them with objective lenses with apertures to f/2.0 or better. However, their bulk and weight do not allow them always to be carried about one's person. Furthermore, for most people, their cost is prohibitive.
Photographic equipment incorporated into mobile phones or tablets is also known in the prior art. Most mobile phones or tablets currently on sale now incorporate an image and video capture system. Because of their use, mobile phones or tablets are not very bulky, meaning that they can be carried around everywhere in a pocket of a garment or of a bag. However, because of their compactness, the inbuilt camera also needs to be small in size. The sensor is therefore small in size, its diagonal generally being less than 7 mm, and the optical system is small also, the aperture thereof being at most f/2.0. The small size of the sensor means low sensitivity but a greater depth of field, all of this in comparison with a reflex or DSLR camera. Because the optical system needs to be small in size, its aperture will be small. The camera built into the phone or the tablet will therefore be all the less able satisfactorily to record very low lighting scenes. The exposure time seen with this equipment is of the order of 1/15s under these conditions. This is longer than the maximum time defined hereinabove that will allow a photo to be taken without movement blur. By default, in low lighting situations, the phone or the tablet will therefore have a tendency to use a flash in order to capture an image without movement blur. In the case of high resolution video recording (HD 1080p video format or better), this 1/15s exposure time is not compatible with a refresh rate of at least 24 images per second.
Devices of the clip-on camera type are also known from the prior art. Before mobile phones were almost systematically fitted with cameras, some manufacturers offered external camera modules to be connected to the phone. These modules incorporated an optical view finder, a VGA camera and a flash. They allowed the camera functionality to be added to the phone but did not allow pictures to be taken with very high resolution under low lighting conditions.
Devices of the action camera type, such as for example the devices marketed by GOPRO® (registered trademark of Gopro, Inc.), are also known from the prior art. These devices are not intended to be carried around everywhere as they are and are not intended to be connected rigidly to a phone or a tablet. Devices of the action camera type are devices intended chiefly for making videos in so-called sports environments. They are designed to be used with a casing. They are not optimized for capturing stationary images in high resolution under low lighting conditions. They are designed to operate remote from their control screen.
Devices of the surveillance camera type connected to a phone or to a tablet via a wireless device are also known in the prior art. These devices are intended to be left at a fixed location, the phone or the tablet being used only for remote monitoring. As they are not physically attached to the phone or to the tablet, they cannot be used hand-held.
Picture taking systems using matrix cameras are also known in the prior art. By way of example, mention may be made of the systems marketed by the following companies: Lytro, Pelican, Raytrix and Toshiba. These systems have significant disadvantages. For example, the Lytro systems have a low-resolution output (1 MP or megapixel) and require post-capture processing in order to obtain a final image. The Pelican systems have a plurality of (sixteen) low-resolution monochrome sensors. The Raytrix ones are very bulky and need to be connected to a computer. They have a moderate resolution (3 megapixels), very high cost and are not widely available to the public. Finally, the Toshiba systems are mainly designed for detecting distance (3D estimation) and are not designed to output high-resolution images.